Anchor Structure and Downhole Plugging Apparatus

ABSTRACT

An anchor structure is disclosed; the anchor structure is set on a mandrel of a downhole plugging apparatus. The anchor structure comprises (a) a clamping device, (b) a plurality of slips and (c) a conical slip holder, wherein the slips are connected to a conical face of the conical slip holder, and (d) a containing space between the clamping device and the slips; the anchor structure further comprises (e) a stroke pressure compensating device which is set within the containing space and can be compressed to provide pressure for the slips, wherein the containing space is designed to be able to accommodate whole or part of the stroke pressure compensating device in the situation that the slips are anchored into a wellbore wall and the clamping device is in contact with the conical slip holder. The containing space would prevent the stroke pressure compensation device from being damaged or extruded completely, and preclude the clamping device from contacting the slips directly when the slips retract.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 to Chinese patent application 201420105505.x filed Mar. 10, 2014, the disclosure of which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

Not Applicable

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to oil-gas production and mineral mining field, particularly, to an anchor structure and a downhole plugging apparatus.

2. General Background of the Invention

In the oil-gas production and mineral mining process, downhole blocking is necessary. Current apparatus of downhole blocking is to adopting a plugging apparatus to block a wellbore, and existing plugging apparatuses are usually composed of a mandrel, an anchor structure and a seal structure that are set on the mandrel. After put into a predetermined position in a well, the existing plugging apparatus would be fixed through squeezing the anchor structure to make it anchored into a wellbore wall. Meanwhile, the anchor structure would squeeze the seal structure, leading to its radial expansion, and then to complete sealing.

However, said exiting downhole plugging apparatuses have such problems such as: they cannot provide a firmly fixing function in the well: their conical slip holders are tend to be damaged; their slips are likely to be damaged or lose efficacy; and, a high degree of tensile strength of the mandrel is required.

By research, the inventor found that:

The reason why existing downhole plugging apparatus cannot provide firmly fixing for drill plugs in well is due to a complex subterranean geological condition which would inevitably cause the wellbore wall being non-circular. In such situation, only a few slips would be anchored into the wellbore wall, the few slips will prevent the clamping device from squeezing the other slips that have not been anchored into the wellbore wall, thus impact the stability of the fixing of downhole plugging apparatus.

At the same time, the stress of the clamping device will be transmitted to the conical slip holder by the few slips that have been anchored into the wall, leading to uneven force on the conical slip holder, with the excessive force acting upon only a few parts of it, it is likely to cause damage to the conical slip holder.

The reason why the slips are easy to be damaged is that: after the slips are anchored into the wellbore wall, the force would transmit from the clamping device to the conical slip holder through the slips, and then to the seal assembly through the conical slip holder, during which process the slips would bear force directly, and the friction between the teeth of the slips and the wall will lead to the damage of the slips.

The high requirement for tensile strength of the mandrel is due to the force will be divided while transmitting from the clamping device to the conical slip holder through the slips, and then to the seal assembly through the conical slip holder, thus majority force would be divided in the radial direction of the mandrel due to the contact area of the slips and the conical slip holder is a inclined surface. For example, if a force of 49KN is required to enable the seal assembly to seal against the wellbore wall to accomplish sealing, since the force from the clamping device will be divided, actually a force of 196KN will be needed from the clamping device, in another word, a tensile force of 196KN will be sustained by the mandrel, which is much higher than that needed by the seal assembly. And this puts forward a higher requirement for the tensile strength of the mandrel. Therefore, the current available mandrels usually have a relatively large inner wall thickness in order to guarantee the tensile strength, which means a relatively small inside diameter for the same type of mandrel; consequently the flow rate will be decreased as well as the production for oil and gas.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature, objects and advantages of the present invention, reference should be had to the following detailed description, read in conjunction with the following drawings:

FIG. 1 is a structural diagram A of a first embodiment of a upper anchor assembly;

FIG. 2 is a structural diagram B of a first embodiment of a upper anchor assembly;

FIG. 3 is a structural diagram A of a second embodiment of a upper anchor assembly;

FIG. 4 is a structural diagram B of a second embodiment of a upper anchor assembly;

FIG. 5 is a structural diagram A of a third embodiment of a upper anchor assembly;

FIG. 6 is a structural diagram B of a third embodiment of a upper anchor assembly;

FIG. 7 is a structural diagram of a downhole plugging apparatus;

FIG. 8 is a state diagram of a downhole plugging apparatus in a setting state;

FIG. 9 is a structural diagram of a conical slip holder;

FIG. 10 is a structural diagram of a elastic slip race;

Wherein the names of related components are as below: 1—mandrel, 2—upper lock nut, 3—stripper ring, 4—slips, 5—conical slip holder, 6—middle rubber sleeve, 7—end rubber sleeve, 8—lower lock nut, 9—elastic slip race, 10—vertical surface, 11—step surface, 12—spring, 13—release lever, 14—wellbore wall, 51—conical surface, 52—separate protrusions, 101—flow channel.

DETAILED DESCRIPTION OF THE INVENTION

One objective of the present invention is to provide an anchoring structure that can be fixed firmly and steadily in a wellbore, a conical slip holder that is durable, a plurality of slips that are less likely to be damaged or lose efficacy, and a mandrel that has a low requirement for the tensile strength could be used for wells with small diameters.

Another objective of the present invention is to provide a downhole plugging apparatus that can be fixed firmly and steadily in a wellbore wall, the conical slip holder that is durable, the slips that are less likely to be damaged or lose efficacy, and the mandrel that has a low requirement for the tensile strength and can be used for wells with small diameters.

Objectives of the present invention are realized through the following:

The anchor structure is arranged on the mandrel, it comprises a clamping device, the plurality of the slips, the conical slip holder and a stroke pressure compensating device; the slips are in contact with a conical surface of the conical slip holder, and there is an containing space between the clamping device and the slips; the anchor structure also comprises the stroke pressure compensating device which is set within the containing space and can be compressed to provide pressure for the slips; the containing space is designed to be able to accommodate whole or part of the stroke pressure compensating device when the slips are anchored into the wellbore wall and the clamping device is in contact with the conical slip holder.

Accordingly, the stroke pressure compensating device is advantaged as follows: when the well is non-circular, part of the slips could be anchored into the wellbore wall, nevertheless, the stroke pressure compensating device could enable the clamping device to continue to squeeze remaining slips to the anchor into the wellbore wall, which contribute to a more firmly fixing of the downhole apparatus in the non-circular wellbore.

Preferably, after all the slips are anchored into the wellbore wall, the force would be transmitted from the clamping device to the conical slip holder through the stroke pressure compensating device and through all the slips instead of only a few of them, thus the conical slip holder would bear force evenly and would not be damaged.

After all the slips anchored into the wellbore wall, the clamping device would continue to squeeze the stroke pressure compensating device and force a bottom of the stripper ring or a lower lock nut contact a bottom of the conical slip holder, then the force will be transmitted directly to the conical slip holder from the clamping device, during which the slips will not bear stress directly, thus the friction between teeth of the slips and the wellbore wall could be avoided, and the slips would not be damaged.

After all the slips anchored into the wellbore wall, the clamping device continues to squeeze the stroke pressure compensating device till a bottom of the clamping device contact the bottom of the conical slip holder, and the force will be transmitted directly to the conical slip holder from the clamping device instead of transmitted through the slips; and the force from the clamping device will be transmitted parallel to an axis of the mandrel without being divided. For example, if a seal assembly needs the force of 49 N to be anchored into the wellbore wall, according to one embodiment of the present invention, only the force of 49 N is all that needed to complete the sealing because the force is transmitted from the clamping device to the conical slip holder directly, the force sustained by mandrel would be only 49 N as well. Therefore the inside diameter for the mandrel could be enlarged without changing the outside diameter, so is the flow rate and the well production; besides, this new structure could enhance a sealing performance since the force bore by the seal assembly may be increased.

By setting up the stroke pressure compensating device, the downhole plugging apparatus can be firmly fixed, the conical slip holders are protected from damages, the slips are prevented from being damaged or lost efficacy, and a high degree of the tensile strength of the mandrel is not required. However the stroke pressure compensating device alone cannot ensure a 100% stability, especially in the wells with relative small diameters.

When the anchor structure with only the stroke pressure compensating device works in small diameter wells, a small stroke will be enough for the slips to be anchored into the wellbore wall. In such case there would be a relatively large gap between the clamping device and the conical slip holder. In order to have the clamping device contacting the conical slip holder, the stroke pressure compensating device needs to be compressed continuously, and the high pressure would destroy the stroke pressure compensating device and extrude the damaged stroke pressure compensating device from the space between the clamping device and the slips. Without the stroke pressure compensating device, the slips cannot have sufficient pressure to be anchored into the wellbore wall, and the slips would retract and contact with clamping device directly under the pressure from the conical surface of the conical slip holder and the wellbore wall, meanwhile the pressure from the clamping device will be transmitted to the conical slip holder through the slips, and then from the conical slip holder to the seal assembly, in such case, the transmitting way of the pressure goes back to the same as that of the conventional downhole plugging apparatus, followed by damaging to the conical slip holder as well as the slips, and inevitably the mandrel will be fractured as well.

Accordingly, the anchor structure may be adjusted by increasing the containing space of the stroke pressure compensating device, which means the containing space can accommodate the whole or partial stroke pressure compensating device after the slips are anchored into the wellbore wall and clamping device contact with the conical slip holder.

According to one embodiment of the present invention, when the clamping device is in contact with the conical slip holder, the whole or part of the stroke pressure compensating device can still be accommodated and can continue to provide pressure for the slips; the slips would not retract or contact the clamping device.

Preferably, dimensions of the slips and the conical slip holder are designed to ensure that the distance between the slips and clamping device is larger than that between the conical slip holder and the said clamping device when the slips are in contact with the wall.

Accordingly, a length of the conical slip holder is increased or a length of the slips is decreased to ensure followed: when the slips are in contact with the conical slip holder, the distance between the slips and the clamping device is larger than that between the conical slip holder and the clamping device, thus when the conical slip holder is in contact with the clamping device, the containing space between the clamping device and the slips still exists, and the stroke pressure compensating device in the containing space can continue to provide pressure to the slips. Though the decreasing of the containing space will lead to part of the stroke pressure compensating device being damaged and extruded, the remaining part of the stroke pressure compensating device can be accommodated by the containing space and provide the slips with pressure.

Preferably, end faces of the said clamping device which are close to the slips, including faces of an external vertical surface and an internal step surface of the clamping device in radial direction, and space among a vertical surface, a step surface and the slips constitute the containing space; the radial length of the step surface is smaller than the minimum distance between the slips and the mandrel.

Accordingly, the end face of the clamping device has been improved. by providing the step surface and the vertical surface on the end face of the clamping device. When the step surface contacts with the conical slip holder, the containing space between the vertical surface and the slips would still exists, wherein the stroke pressure compensating device in the containing space can provide the slips with the pressure continuously. Though decreasing of the containing space will lead to part of the stroke pressure compensating device being damaged or extruded, the remaining part of the stroke pressure compensating device can still be accommodated by the containing space and provide the slips with the pressure.

The radial length of the step surface is less than a minimum distance between the slips and the mandrel, so that the step surface would not contact the slips.

Preferably, the stroke pressure compensating device could be an elastic slip race.

While operating, the clamping device is in contact with the conical slip holder, the elastic slip race is compressed and it provides the slips with the pressure. In another situation, when the clamping device contacts with the conical slip holder, part of the elastic slip race is destroyed, the remaining elastic slip race in the containing space would continue to provide slips with pressure.

Preferably, the stroke pressure compensating device could be the elastic slip race; or the stroke pressure compensating device comprising the elastic slip race, a spring or a constant pressure plunger valve, two ends of the spring or the constant pressure plunger valve connect to the vertical surface and the elastic slip race, respectively; when the spring or the constant pressure plunger valve is compressed to its limitation, its length is less than or equal to the axial distance between the vertical surface and the step surface.

While operating, according to one embodiment of the present invention, the clamping device contacts with the conical slip holder, the elastic slip race, the spring or the constant pressure plunger valve will be compressed in the containing space, which provides the slips with the pressure; alternatively, when the clamping device contacts the conical slip holder, the elastic slip race will be destroyed and extruded, and the remaining spring or the constant pressure plunger valve would provide the slips with the pressure; when the spring or the constant pressure plunger valve is compressed to its limitation, the compressed length is less than or equal to the distance between the vertical surface and the step surface in the axial direction, which prevents the spring or the constant pressure plunger valve from losing efficacy.

Yet another objective of the present invention is accomplished by the followed:

The downhole plugging apparatus comprises the mandrel, the seal assembly set on the mandrel, a upper anchor assembly and a lower anchor assembly connected to the upper and the lower end of the mandrel respectively; the upper anchor assembly and the lower anchor assembly consist of a anchor structures; the anchor structure includes the clamping device, multiple slips and the conical slip holder; the slips are in contact with the conical surface of the conical slip holder; and, the containing space between the clamping device and the slips. The anchor structure also includes the stroke pressure compensating device, which is designed in the containing space and can be compressed to provide the slips with the pressure; the containing space can accommodate the whole or part of the stroke pressure compensating device when the slips are anchored into the wellbore wall and the clamping device contact with the conical slip holder. In the upper anchor assembly, the clamping device consists of the stripper ring; in the lower anchor assembly, the clamping device consists of the lower lock nut connected to a thread on the bottom end of the mandrel.

Preferably, the dimensions of the slips and the conical slip holder are designed to ensure that the distance between the slips and the clamping device is larger than that between the conical slip holder and the clamping device when the slips are in contact with the wall.

Preferably, the end faces of the clamping device which are close to the slips, including the faces of the external vertical surface and the internal step surface of the clamping device in radial direction, and space among the vertical surface, the step surface and the slips constitute the containing space; the radial length of the step surface is smaller than the minimum distance between the slips and the mandrel.

Preferably, the stroke pressure compensating device could be the elastic slip race, the spring or the constant pressure plunger valve.

Preferably, the stroke pressure compensating device is the elastic slip race; or the stroke pressure compensating device comprises the elastic slip race, the spring or the constant pressure plunger valve, the two ends of the spring or the constant pressure plunger valve connect to the vertical surface and the elastic slip race respectively, when the spring or the constant pressure plunger valve is compressed to its limitation, the length is less than or equal to the axial distance between the vertical surface and the step surface.

Accordingly, the advantages of this utility model are:

1. With the stroke pressure compensating device, all of the slips are able to be anchored into the wellbore wall, which guarantee a more stable and reliable fixing of the downhole plugging apparatus in the non-circular well, and the conical slip holder is less likely to be damaged.

2. With the stroke pressure compensating device, the friction between the teeth of the slips and the wellbore wall could be avoided.

3. When sealing is accomplished, with the stroke pressure compensating device, the force born by the mandrel will be decreased, thus the inside diameter of the mandrel could be relatively enlarged without changing the outside diameter, which means the flow rate will be increased as well as the production.

4. With the containing space, the stroke pressure compensation device can be prevented from being damaged or extruded completely, and the clamping device can be prevented from contacting the slips directly when the slips retract.

5. The radial length of the step surface is less than the minimum distance between the slips and the mandrel, thus direct contact between the step surface and the slips can be avoided.

Description of Specific Embodiment

To provide a better understanding of the utility model patent, a clear and complete description combined with the figures of the embodiments of the utility model patent will be presented in this section.

Embodiment 1

As is shown in FIG. 1 to FIG. 6, the anchor structure is provided on the mandrel 1 and used as the downhole plugging apparatus, which comprises the clamping device, the plurality of the slips 4 and the conical slip holder 5, the slips 4 are in contact with the conical surface of the conical slip holder 5, and the containing space between the clamping device and the slips 4; the anchor structure also comprises the stroke pressure compensating device which is provided within the containing space, wherein the stroke pressure compensating device can be compressed to provide the pressure for the slips 4; the containing space is designed to be able to accommodate the whole or part of the stroke pressure compensating device when the slips are anchored into the wellbore wall and the clamping device is in contact with the conical slip holder 5.

According to the current embodiment, the stroke pressure compensating device could be made of the elastic rubber, the spring, the constant pressure plunger valve or other elements with elasticity.

In the upper anchor assembly, the clamping device is the stripper ring 3; in the lower anchor assembly, the clamping device is the lower lock nut 8.

FIGS. 1, 3 and 5 show a non-operating state for several different anchor structures of the upper anchor assembly of the downhole plugging apparatus.

FIGS. 2, 4 and 6 show an operating state for several different anchor structures of the upper anchor assembly of the downhole plugging apparatus.

The working principle of this embodiment is as followed:

When the well is out-of-round, only a few of the slips 4 could be anchored into the wellbore wall 14. However, a stroke pressure compensating device could enable the stripper ring 3 and the lower lock nut 8 to continue to squeeze the slips 4 that have not been anchored into the wellbore wall 14 and force all of them anchored into the wall, thus the downhole plugging apparatus could be fixed firmly and steadily.

Preferably, after all the slips 4 are anchored into the wellbore wall 14, the force would transmit from the stripper ring 3 and the lower lock nut 8 to the conical slip holder 5 through all the slips 4, with even force on the conical slip holder 5, there would be no damage to the conical slip holder 5.

After all the slips 4 are anchored into the wellbore wall, the stripper ring 3 and the lower lock nut 8 would continue to squeeze the stroke pressure compensating device till the bottom of the stripper ring 3 or the lower lock nut 8 contact the bottom of the conical slip holder 5, the force of the stripper ring 3 and the lower lock nut 8 will be transmitted directly to the conical slip holder 5, since the slips 4 will not bear force directly, the friction between the teeth of the slips 4 and the wall 14 can be avoided and thus no damage to the slips 4.

After the slips 4 are anchored into the wellbore wall, the stripper ring 3 or the lower lock nut 8 would continue to squeeze the stroke pressure compensating device till the bottom of the stripper ring 3 or the bottom of lower lock nut 8 contact the bottom of the conical slip holder 5, then the force will be transmitted directly to the conical slip holder 5 from the stripper ring 3 or the lower lock nut 8 instead of transmitting through slips 4; the force of the stripper ring 3 and the lower lock nut 8 will be transmitted parallelly to the axis of the mandrel 1 without being divided, thus all the force will be transmitted to the sealing assembly. When sealing is completed, the force bore by the mandrel 1 is greatly decreased, and the inside diameter of the flow hole 101 could be enlarged when outside diameter is the same, which means the flow rate will be increased as well as the production.

Accordingly, when the slips 4 are anchored into the wellbore wall 14 and the clamping device contacts the conical slip holder 5, the containing space can still accommodate the whole or part of the stroke pressure compensating device, and the slips would not contact the clamping device when they retract.

Embodiment 2

As shown in FIG. 3 and FIG. 4, sizes of the slips 4 or the conical slip holder 5 are designed to ensure that when the slips 4 are in contact with the wall, the distance between the slips 4 and clamping device is still larger than that between the conical slip holder 5 and the clamping device. according to the current embodiment, the length of the conical slip holder 5 is increased or the length of the slips 4 is decreased, thus when the slips are in contact with the conical slip holder 5, the distance between the slips 4 and the clamping device is larger than that between the conical slip holder 5 and the clamping device. Therefore, when the conical slip holder 5 is in contact with the slips 4, the containing space between the clamping device and the slips 4 still exists, and the stroke pressure compensating device in the containing space can provide the slips 4 with the pressure continuously. Though the decreasing of the containing space will lead to part of the stroke pressure compensating device being damaged or extruded, the remaining part of the stroke pressure compensating device can be accommodated by the containing space and provide the slips 4 with the pressure.

According to the current embodiment, the stroke pressure compensating device is the elastic slip race 9, the elastic slip race 9 is made of elastic rubber. While operating, part of the elastic slip race 9 could be destroyed and extruded, and the remaining elastic slip race in the containing space will provide the slips 4 with pressure.

Embodiment 3

As shown in FIGS. 1, 2, 5 and 6,

the end faces of the said clamping device which are close to the slips 4, including the faces of the external vertical surface 11 and the internal step surface 10 of the clamping device in the radial direction; and the space among the vertical surface 11, the step surface 10 and the slips 4 constitute the containing space; the radial length of the step surface 10 is smaller than the minimum distance between the slips 4 and the mandrel.

In the upper anchor assembly, the step surface 11 and the vertical surface 10 are on the stripper ring 3; in the lower anchor assembly, the step surface 11 and the vertical surface 10 are on the lower lock nut 8.

Accordingly, the end faces of the clamping device has been improved by; provided with the step surface 11 and the vertical surface 10 on the end face of the clamping device. When the step surface contacts with the conical slip holder 5, the containing space between the vertical surface 10 and the slips 4 would still exists, and the stroke pressure compensating device in the containing space can provide the slips 4 with the pressure continuously. Though the decreasing of the containing space will lead to part of the stroke pressure compensating device being damaged and extruded, the remaining part of the stroke pressure compensating device can still be accommodated by the containing space and provide the slips with the pressure.

To prevent the step surface 11 from contacting with the slips 4, the radial length of the step surface is designed to be less than the minimum distance between the slips 4 and the mandrel 1.

According to the current embodiment, the stroke pressure compensating device is the elastic slip race 9 as shown in FIG. 10; or the stroke pressure compensating device comprises the elastic slip race 9, the spring 13 or the constant pressure plunger valve, the two ends of either the spring 13 or the constant pressure plunger valve connect with the vertical surface 10 and the elastic slip race 9, respectively; when the spring 13 or the constant pressure plunger valve is compressed to its limitation, its length is less than or equal to the axial distance between the vertical surface 10 and the step surface 11.

As shown in FIG. 1 and FIG. 2, the stroke pressure compensating device is the elastic slip race 9. While setting, some parts of the elastic slip race 9 would be destroyed, while the remaining elastic slip race 9 stays in the containing space between the vertical surface 10 and the end surface of the slips 4 and provides the slips 4 with pressure.

As shown in FIG. 5 and FIG. 6, the stroke pressure compensating device is a combination of the spring 13 and the elastic slip race 9. While setting, the elastic slip race 9 could be destroyed and extruded, while the remaining spring 13 stays in the containing space between the vertical surface 10 and the end surface of the slips 4 and provides the slips 4 with the pressure.

Alternatively, the constant pressure plunger valve or other elastic devices can replace the spring 13.

Embodiment 4

As shown in FIG. 7 and FIG. 8, the downhole plugging apparatus comprises the mandrel, the seal assembly set on the mandrel, the upper anchor assembly and the lower anchor assembly that are connected to the upper and the lower end of the seal assembly respectively; the upper anchor assembly and the lower anchor assembly consist of the anchor structures; the anchor structures could be any of that in the embodiments 1 to 3.

The working principle of this embodiment is followed: a release lever 13 connects with an external claw by threading, the claw consists of the clamping device that contacts with the upper anchor assembly. The clamping device would apply the force on the upper anchor assembly, then both of the upper anchor assembly and the lower anchor assembly would squeeze the seal assembly, so that the seal assembly would be compressed axially and expand radially. As a result, the upper anchor assembly and the lower anchor assembly would be anchored into the wellbore wall 11, thus the seal assembly can be sealed firmly against the wellbore wall and the downhole plugging apparatus can be fixed. Then the release lever 13 will be cut when being pulled and the downhole plugging apparatus will be left in the well.

Embodiment 5

The upper anchor assembly is provided with a threaded hole inside the upper lock nut 2 so that the upper lock nut 2 could be connected with the mandrel 1 by threads.

The mandrel 1 is sleeved by the stripper ring 3, which could move axially along the mandrel 1; the end face of the stripper ring 3 that contacts with the upper lock nut 2 is plane.

As shown in FIG. 9, the mandrel 1 is sleeved with the conical slip holder 5, which could travel axially along the mandrel 1 when bearing force. The end face of the conical slip holder 5 facing the stripper ring 3 is a conical surface 51 on which separate protrusions 52 are arranged evenly; and an other end face of the conical slip holder 5 is plane and contacts with the seal assembly. Each of the slips 4 is in contact with the conical surface 51 and is set between two adjacent separate protrusions 52.

In the lower anchor assembly, there is a threaded hole inside the lower lock nut 8 so that the lower lock nut 8 could be connected with the mandrel 1 by threading.

While operating, first, the two-stage mandrel for rubber sleeve pressurization is fixed via the release lever 13. Then the stripper ring 3 is pressed, and the stripper ring 3 together with the lower lock nut 8 would push the slips 4 travelling on the conical surface 51 of conical slip holder 5. At the same time, the force will be transmitted to the seal assembly through the conical slip holder 5, so that the seal assembly will be compressed axially and expanded radially, and the teeth of the slips 4 will be anchored into the wellbore wall 14, keeping the seal assembly in a state of compression and accomplishing the sealing.

The said seal assembly consists of the middle rubber sleeve 6 that sleeves the two-stage mandrel for rubber sleeve pressurization and the end rubber sleeves 7 that are arranged at both ends of the middle rubber sleeve 6.

Although each of the embodiment have been described in great detail, it is to be understood that numerous modifications, variations and adaptations may be made to the particular embodiments of the invention described above without departing from the scope of the invention which is defined in the claims. 

What is claimed is:
 1. An anchor structure provide to a mandrel of a downhole plugging apparatus, comprising: a clamping device, a plurality of slips 4, a conical slip holder 5 and a stroke pressure compensating device, wherein the slips are connected with a conical surface of the conical slip holder 5, and a containing space is provided between the clamping device and the slips 4; a stroke pressure compensating device in the containing space, which provide a pressure for the slips when it is compressed, wherein the containing space is designed to be able to accommodate whole or part of the stroke pressure compensating device when the slips are anchored into a wellbore wall and the clamping device is in contact with the conical slip holder
 5. 2. The anchor structure from claim 1, further comprises: a size of the slips 4 or the conical slip holder 5 is designed to ensure that when the slips 4 are in contact with the wellbore wall, a distance between the slips 4 and the clamping device is larger than that between the conical slip holder 5 and the clamping device.
 3. The anchor structure from claim 1, further comprises: the containing space is formed by end faces of the clamping device which are close to the slips 4, including faces of an external vertical surface 10 and an internal step surface 11 of the clamping device in a radial direction, and space among the vertical surface 10, the step surface 11 and the slips 4 constitute the containing space; a radial length of the step surface 11 is smaller than a minimum distance between the slips 4 and the mandrel
 1. 4. The anchor structure from claim 2, further comprises: the containing space is formed by end faces of the clamping device which are close to the slips 4, including faces of an external vertical surface 10 and an internal step surface 11 of the clamping device in a radial direction, and space among the vertical surface 10, the step surface 11 and the slips 4 constitute the containing space; a radial length of the step surface 11 is smaller than a minimum distance between the slips 4 and the mandrel
 1. 5. The anchor structure from claim 1, wherein the stroke pressure compensating device is an elastic slip race
 9. 6. The anchor structure from claim 3, wherein the stroke pressure compensating device is an elastic slip race
 9. 7. The anchor structure from claim 3 further comprises: the stroke pressure compensating device consists of the elastic slip race 9, a spring 13 or a constant pressure plunger valve, wherein both ends of the spring 13 or the constant pressure plunger valve are connected to the vertical surface 10 and a elastic slip race 9, respectively; when the spring 13 or the constant pressure plunger valve is compressed to its limitation, its length is less than or equal to an axial distance between the vertical surface 10 and the step surface
 11. 8. A downhole plugging apparatus, comprising: a mandrel, a seal assembly which is provided on the mandrel 1, an upper anchor assembly and a lower anchor assembly that are connected to a upper end and a lower end of the mandrel 1, respectively; the upper anchor assembly and the lower anchor assembly consist of anchor structures including a clamping device, a plurality of slips 4 and a conical slip holder 5, wherein the slips 4 contact to a conical surface of the conical slip holder 5, and a containing space is provided between the clamping device and the slips 4; the anchor structures also include a stroke pressure compensating device, which is designed in a containing space and can be compressed to provide the slips with a pressure 4; the containing space can accommodate whole or part of the stroke pressure compensating device when the slips 4 are anchored into a wellbore wall and the clamping device contacting with the conical slip holder 5; the clamping device of the upper anchor assembly comprises a stripper ring 3; the clamping device of the lower anchor assembly comprises a lower lock nut 8 which is connected to thread at a bottom end of the mandrel
 1. 9. The downhole plugging apparatus from claim 8, further comprising: size of the slips 4 and the conical slip holder 5 are designed to ensure that a distance between the slips 4 and the clamping device is larger than that between the conical slip holder 5 and the clamping device when the slips 4 are in contact with the wellbore wall.
 10. The downhole plugging apparatus from claim 8, further comprising: the containing space is formed by (1) end faces of the clamping device which are close to the slips 4, including faces of an external vertical surface 10 and an internal step surface 11 of the clamping device in radial directions, and (2) space among the vertical surface 10, the step surface 11 and the slips 4; a radial length of the step surface 11 is smaller than a minimum distance between the slips 4 and the mandrel
 1. 11. The downhole plugging apparatus from claim 9, further comprising: the containing space is formed by (1) end faces of the clamping device which are close to the slips 4, including faces of an external vertical surface 10 and an internal step surface 11 of the clamping device in radial directions, and (2) space among the vertical surface 10, the step surface 11 and the slips 4; a radial length of the step surface 11 is smaller than a minimum distance between the slips 4 and the mandrel
 1. 12. The downhole plugging apparatus from claim 8, further comprising: the stroke pressure compensating device could be a elastic slip race 9, a spring 13 or a constant pressure plunger valve.
 13. The downhole plugging apparatus from claim 9, further comprising: the stroke pressure compensating device could be a elastic slip race 9, a spring 13 or a constant pressure plunger valve.
 14. The downhole plugging apparatus from claim 10, further comprising: the stroke pressure compensating device consists of a spring 13, a constant pressure plunger valve or a elastic slip race 9, both ends of the spring or the constant pressure plunger valve are connected to the vertical surface 10 and the elastic slip race 9, respectively; when the spring 13 or the constant pressure plunger valve is compressed to its limitation, its length is less than or equal to an axial distance between the vertical surface 10 and the step surface
 11. 